1. Field of the Invention
The present invention relates to a turbine power plant system that responds to a loss of electrical load to rapidly reduce the turbine generator output and following a system load disturbance operate at a sustained reduced load.
2. Description of the Prior Art
Electric power from a turbine generating plant may be transmitted over many miles of electric cable; and in many instances more than one geographically spaced turbine power plant may be connected to, or feeding the same transmission line, with each plant contributing to the total load. It is customary to transmit each phase of a three phase alternating current, for example, over transmission lines which are spaced apart sufficiently so that damage to one line by an extraneous cause, such as lightning, is unlikely to damage an adjacent cable. However, in the event that one of the transmission lines is broken or shorted close to the generating plant, the electrical load produced by such plant is suddenly reduced. This results in the unit's tending to speed up, which may taken it out of synchronism with the other plants.
In the past, this problem of short circuits or line faults, which resulted in a partial load reduction, was solved by taking the plant off the line; that is, tripping the turbine. However, in such an event where other units are contributing load to the same transmission line, the taking of one plant off line may cause the other turbine power plants to trip. Another proposed way of minimizing the likelihood of instability due to a line fault is to utilize one or more spare transmission lines so that the detection of a fault on one line causes a transfer of the output to a spare line.
Another way of preventing a turbine power plant from getting out of synchronism upon the occurrence of a partial load reduction is to effect what is commonly known as "fast valving", which involves a rapid momentary reduction in the admission of steam to the turbine power plant in response to the detection of a line fault to keep the turbine from tripping. In such power plants, where the turbine has more than one pressure stage, the low pressure stage or section provides approximately 70% of the driving power of the turbine. Thus, the governor valves, which control the flow of steam to the high pressure section in accordance with the desired load, would not cause sufficient rapid reduction of the turbine output to prevent the tripping of the turbine even if they were all fully closed rapidly. The steam to the lower pressure stages is conducted from the exhaust of the high pressure section through steam reheating apparatus and a plurality of intercept valves to the lower pressure turbine section. The interceptor valves are normally operated to a fully open position to admit the flow of steam from the high to the lower pressure sections. The rapid closing and opening of the interceptor valves until the line fault is removed or the proper load is restored as described in U.S. Pat. No. 3,843,437, is effective to prevent the tripping of the turbine in the case of a partial load reduction. However, such a transient type situation causes a pressure buildup upstream of the interceptor valves in response to the closure, and a sudden rush of steam in response to the opening. This may cause pressure shocks, temperature swings on the turbine blading, and "secondary swings" of power on the grid system.
In the past, it has been proposed to rapidly close the interceptor valves and then open them rapidly for a portion of their stroke and then slowly open such valves until they are fully open. This of course cuts off the steam supply momentarily to the lower pressure turbine stages and minimizes the sudden rush of steam by restricting the rapid reopening. Also, it has been proposed to rapidly close a portion of the governor valves in order to reduce the steam flow to the HP turbine.
In present electrohydraulic control systems, either digital or analog, there is usually provided an electrical reference demand signal which controls the total steam flow to the turbine by controlling the governor valves in either a single or a sequential mode. However, for adequacy of normal control the rate of change of such signal is limited. Also, such control systems normally operate in an automatic mode with an impulse pressure feedback loop in service to maintain a constant load on the system. Thus, it is important that a "fast valving" system be compatible with such electrohydraulic control systems so that "fast valving" in response to a line fault, does not affect the proper operation of the turbine control system; and that such system is restored to control the normal operation of the turbine at a predetermined sustained reduced load with such transition being effected without abrupt changes in valve position.
U.S. Pat. No. 3,848,138 suggests certain of the previously discussed concepts of opening and closing the intercept and governor valves; and presents in detail the history of fast valving. Also, systems have been proposed for fast valving and for stabilizing a steam generator after run back of the load in U.S. Pat. Nos. 3,601,617 and 3,609,384. Although, such patents have proposed various systems for fast valving, a system that is capable of utilizing the described well-known fast valving concepts, yet is readily adaptable to modern electrohydraulic turbine control systems, eliminating the effect of the impulse pressure fluctuations, closing one portion and decreasing the opening a predetermined of another portion of the governor valves independent of the reference demand signal to overcome the inherent relative slow rate of change of the reference demand signal, or which provides for rapid restoration of a decreased total flow of steam to the lower turbine pressure stages regardless of the sticking or slow operation of one or more of the interceptor valves, and which provides for a smooth transition to normal turbine control at a sustained reduced load has not been shown insofar as is known.
Thus, it is desirable to provide an improved fast valving system that provides the necessary protection to the steam turbine generator in response to a load fault signal, and overcomes the limitations of, and is comparable with a modern electrohydraulic system.